Optical system for a thin, low-chin, projection television

ABSTRACT

A micro-mirror based projection display system in an enclosure with minimum chin and depth measurements is disclosed. A solid-state laser light source generates light of multiple primary colors that is modulated by a digital micro-mirror device. The projection optics of the system include a telecentric rear group of glass lenses with spherical surfaces, followed by a pair of aspheric lenses formed of plastic. A folding mirror is disposed between the aspheric lenses, to reduce the depth of the enclosure, and an aspheric mirror projects the image onto a TIR Fresnel projection screen. The aspheric lenses are magnifying, to reduce the magnification required of the aspheric mirror, and the aspheric lenses and mirror are clipped to reduce enclosure volume.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

This invention is in the field of projection display systems, and ismore specifically directed to the arrangement of optical elements insuch a display system.

As is evident from a visit to a modern electronics store, the number offlat-panel (i.e., non-CRT) televisions has vastly increased in recentyears, while the purchase price for such sets continues to fall. Thistremendous competition is due in large part to the competingtechnologies for the display of high-definition television content. Asknown in the art, three major current display technologies forflat-panel televisions include liquid-crystal display (LCD), plasmadisplay, and digital micromirror (DMD) based displays. Themicromirror-based displays, and some LCD displays, are projectiondisplays, in that a light source illuminates a spatial light modulatorformed by the micromirror or LCD panel, with the modulated light thenoptically projected to a display screen. Plasma displays, on the otherhand, are not projection displays; rather, each pixel at the displayscreen includes red, green, and blue phosphors that are individuallyexcitable by way of argon, neon, and xenon gases, producing the image.Some LCD televisions involve “direct-view” displays, rather thanprojection displays, such that the liquid crystal elements are at thedisplay screen and are directly energized to produce the image.

In modern micromirror-based projection displays, such as DLP® projectiondisplays now popular in the marketplace using technology developed byTexas Instruments Incorporated, a digital micromirror device spatiallymodulates light from a light source according to the content to bedisplayed. An optical “engine”, which includes lens and mirror elements,projects the modulated light onto the display screen. As known in theindustry, micromirror-based projection displays are advantageous fromthe standpoint of brightness, clarity, and color reproduction, ascompared with other flat-panel televisions and displays. In addition,the use of micromirror spatial light modulators enable higher-speedmodulation of light than many LCD systems, and micromirror-based systemshave been observed to be extremely reliable over time.

However, conventional micromirror-based projection systems typicallyrequire larger “form factor” enclosures, than do LCD and plasmaflat-panel systems of similar screen size and resolution. Two importantmeasures of the enclosure for flat-panel display systems are referred toin the art as the “chin” dimension and the “depth” of the case. FIG. 1 aillustrates the conventional definition of the “chin” of a flat-paneltelevision, while FIG. 1 b illustrates the “depth” of the system.

As shown in the front elevation view of FIG. 1 a, display screen 2 ishoused within enclosure 4. The portion of enclosure 4 that extends belowscreen 2 constitutes the “chin” of the display system. FIG. 1 aillustrates dimension CHIN as the distance from the bottom edge ofscreen 2 to the bottom of enclosure 4. FIG. 1 b illustrates, inconnection with a side view of enclosure 4, the dimension DEPTH as themeasurement between the front of enclosure 4 and the back of a rear-wardextending portion of enclosure 4. In micromirror-based projectiondisplay systems, the system components of the light source, digitalmicromirror, and the system projection optics, reside within the “chin”and the rearward extending portions of enclosure 4.

Consumers are attracted to televisions and display systems that arethin, from front to back, and for which the enclosure only minimallyextends beyond the dimensions of the display screen itself. Indeed, ithas been observed that the consumer buying decision is often based onthe size of the enclosure for a given screen size. As mentioned above,the enclosures of modern plasma and direct-view LCD display systems cantypically involve minimal chin and depth, because they are not rearprojection systems and as such do not require enclosure of the lightsource, modulator, and projection optics required by projection systems,especially conventional micromirror-based systems. As such, theseconventional micromirror-based projection systems are at a competitivedisadvantage in the marketplace in this regard. And therefore, it isdesirable for micromirror-based projection systems to also minimize thechin and depth of their enclosures, to attain and preserve market share.

In addition to the physical volume required for enclosures of projectiondisplay systems such as those based on micromirrors, other constraintsalso have resulted in substantial chin and depth dimensions. One suchconstraint is due to the TIR (Total Internal Reflection) Fresnel displayscreens that are now commonly used in projection display systems. Asknown in the art, TIR Fresnel display screens are capable of receivinglight at a relatively steep angle from the normal, and of directing thatlight into the direction normal to the display screen, analogous toFresnel lenses as used in traffic lights and lighthouses. Thisconstruction permits the source of the projected light to resideoff-axis with the display screen, which greatly reduces the depth ofprojection display systems. FIG. 1 c shows the rear projection of animage from source 8 (which may be a plane mirror, for example) todisplay screen 2, which is constructed as a conventional TIR Fresneldisplay screen. The angle of incidence of light from source 8 to thebottom of screen 2 is at a minimum angle φ_(m) from the normal, whilethe angle of light from source 8 to the top of screen 2 is at a maximumangle of incidence φ_(x). It has been observed that, for conventionalTIR Fresnel display screens, the minimum angle of incidence φ_(m) shouldbe above 50° from the normal, to avoid flare and reduced contrast inportions of the displayed image. However, in order to achieve such alarge minimum angle of incidence, it is therefore often necessary toconstruct an enclosure having substantial “chin”, as evident from FIG. 1c. In addition, if a plane mirror is used as source 8, to reflect theprojected image to display screen 2, as shown in the conventional systemof FIG. 1 c, the minimum angle constraint commonly requires the heightof this mirror to on the order of one-half the vertical dimension ofdisplay screen 2, especially as the depth of enclosure 4 is minimized.

Other design and manufacturing constraints also affect the design ofconventional display “engines” for micromirror-based projectiondisplays. These other constraints involve the nature of the light source(i.e., the “etendue” of the light), the extent of lens groups andnumbers of lenses required to obtain a high resolution and minimumdistortion image at the display.

By way of further background, a current trend in the construction ofprojection display systems is the use of non-telecentric lenses in theprojection optics, between the spatial light modulator and the displayscreen. As known in the art, “non-telecentric” refers to lensarrangements that receive light from an image or source (i.e., themodulator) that is larger than the lenses; as such, the chief rays oflight from various locations of the image are not parallel to, or notcollimated with, one another. The use of non-telecentric lenses ispopular in projection systems because the diameter of the lenses can bemuch smaller than the image or light source. Not only is the physicalsize of the lenses reduced, but the f-number of the lenses required forefficient light transfer is also kept relatively high, further reducingthe cost of the lenses. As known in the art, large lenses of lowf-numbers are relatively expensive to produce, especially forapplications in which high image quality and resolution is important, asin high-definition television. It has also been observed that higherimage contrast is generally attained by non-telecentric projectionlenses. In addition, display systems using micro-mirror based spatiallight modulators in combination with non-telecentric lenses can omit the“total internal reflection” (TIR) prism for separating “on” and “off”pixel light that is otherwise generally necessary with telecentricprojection lens systems. Non-telecentric projection lenses are thuspopular in modern projection display systems.

By way of further background, however, non-telecentric projection lensesare known to present certain limitations in projection display systems.Defocus caused by thermal or alignment effects at the SLM plane is madeevident as dramatic magnification changes in the displayed image(“overfill” or “underfill”) in systems using non-telecentric projectionlenses, even if the f-number of the projection lens group is relativelyhigh (slow).

Another trend in the design of projection display systems is the use ofwide-angle, high-magnification, aspheric mirrors as the elementreflecting the projected image onto the display screen (e.g., as source8 in the arrangement of FIG. 1 c). It has been reported that the use ofa high magnification aspheric mirror is believed to suppress coloraberration.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a micromirror-basedprojection display system having minimal chin and minimal depth.

It is a further object of this invention to provide such a displaysystem in which distortion is minimized and resolution is maximized.

It is a further object of this invention to provide such a displaysystem that is competitive, from the standpoint of form factor, withmodern LCD and plasma displays.

It is a further object of this invention to facilitate correction ofaberration in the image to be displayed, in a manner consistent with asmall volume form factor.

It is a further object of this invention to provide such a displaysystem that provides more stability in its displayed image qualityrelative to defocus at the SLM plane, for example as may be caused byvariations in temperature or humidity, or by variations in alignmentduring manufacture or use.

Other objects and advantages of this invention will be apparent to thoseof ordinary skill in the art having reference to the followingspecification together with its drawings.

The present invention may be implemented into a projection displaysystem using a laser light source, for example one or more arrays ofsolid-state lasers emitting primary color light incident on a digitalspatial light modulator. The spatial light modulator may be one or moredigital micro-mirror devices (DMDs), or a spatial light modulator ofanother type, such as a liquid crystal on silicon (LCOS) SLM, a hightemperature polysilicon (HTPS) SLM, a transmissive LCD SLM, and adiffractive 1-D SLM. The modulated light is projected to the displayscreen using telecentric projection lenses in a first group, followed bya medium-to-wide angle aspheric projection lens formed of plasticwith >1.0 magnification. A plastic aspheric mirror reflects theprojected image to the display screen.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1 a through 1 c are schematic illustrations of conventionalprojection television displays, illustrating various dimensions therein.

FIG. 2 is a block diagram, in schematic form, of a projection displaysystem constructed according to the preferred embodiment of theinvention.

FIG. 3 a is a schematic elevation view of the arrangement of a lightsource and a digital micromirror device in the projection display systemof FIG. 2, according to the preferred embodiment of the invention, andFIG. 3 b is a flow diagram schematically illustrating the operation ofthe same.

FIGS. 4 a through 4 c are schematic elevation views of projection opticsin the projection display system of FIG. 2, according to the preferredembodiment of the invention.

FIGS. 5 a through 5 d are schematic views of the arrangement of theprojection optics and aspheric mirror in the projection display systemof FIG. 2, according to the preferred embodiment of the invention.

FIG. 6 is a schematic perspective view of the arrangement of portions ofthe projection optics and the aspheric mirror in the projection displaysystem of FIG. 2, according to the preferred embodiment of theinvention, illustrating the various light paths therein.

FIG. 7 is a schematic elevation view illustrating the arrangement of theprojection optics and aspheric mirror with the display screen in theprojection display system of FIG. 2, according to the preferredembodiment of the invention.

FIGS. 8 a and 8 b are plots of magnification over normalized field ofview for lens and mirror elements in the projection display system ofFIG. 2, according to the preferred embodiment of the invention.

FIG. 8 c is a geometric illustration defining normalized field of view,for purposes of describing the performance of the projection systemaccording to the preferred embodiment of the invention.

FIG. 8 d is a plot of distortion over normalized field of view for theprojection system of FIG. 2 constructed according to the preferredembodiment of this invention.

FIGS. 9 a through 9 c are plots of modulation transfer function (MTF)over resolution for a conventional projection display system at varyingtemperature.

FIGS. 9 d through 9 f are plots of MTF over resolution at varyingtemperature for the projection display system of FIG. 2, according tothe preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention will be described in connection with its preferredembodiment, namely as implemented into a micromirror-based projectiontelevision display system, as it is contemplated that this inventionwill be especially beneficial in such a system application. It is alsocontemplated, however, that this invention may be beneficial in otherapplications, and variations on the described television application.Accordingly, it is to be understood that the following description isprovided by way of example only, and is not intended to limit the truescope of this invention as claimed.

FIG. 2 schematically illustrates the functional elements of projectiondisplay system 15 according to the preferred embodiments of thisinvention. The physical arrangement and construction of these elementswill be described in further detail below; the illustration of FIG. 2 ispresented in a functional manner, to provide functional context for thatdetailed description.

As shown in FIG. 2, projection display system 15 includes projectionscreen 12, upon which the displayed image is projected from behind(i.e., from the opposite side of screen 12 from viewer V). In thispreferred embodiment of the invention, screen 12 is preferably a totalinternal reflection (TIR) Fresnel screen, to permit the image to beprojected from an offset position from the center point of screen 12. Inthis case, the displayed image is projected by aspheric mirror 10 frombelow and behind screen 12.

According to this embodiment of the invention, light source 16 directslight of multiple primary colors at spatial light modulator (SLM) 18 inthe conventional manner. Light source 16 is preferably a laser lightsource that directs light of at least three primary colors (e.g., red,green, blue) at SLM 18 in a time-multiplexed manner. As known in theart, other sequential primary color light sources can be constructed asa bulb-and-reflector type of white light source that illuminates arotating color wheel having multiple colored filters; however it iscontemplated, according to this invention, that a laser light sourcewill be preferred, as will be apparent from the following description.

SLM 18 spatially modulates the incident light from light source 16, inresponse to control signals from graphics driver 14. In this preferredembodiment of the invention, SLM 18 is of the digital micromirror (DMD)type, in which a large number of individually controllable micromirrorseach correspond to one pixel of the resulting image, and are eachcontrolled in a time-sequential fashion to selectably reflect light inthe desired light path according to be displayed. DMD devices suitablefor use as SLM 18 are well-known in the art, for example those DMDdevices in the DLP® product family available from Texas InstrumentsIncorporated. While one SLM 18 is illustrated in FIG. 2, and modulateslight of multiple primary colors to produce a full-color displayedimage, it is contemplated that this invention is also applicable tosystems that implement multiple SLM devices 18 (e.g., three SLMs 18, oneeach for red, green, and blue light), as will be evident to thoseskilled in the art having reference to this specification.

Alternatively, SLM 18 may be realized according to other technologies.Such alternative technology SLM devices include liquid crystal onsilicon (LCOS) SLMs, high temperature polysilicon (HTPS) SLMs,transmissive LCD SLMs, and diffractive 1-D SLM devices. While thefollowing description will generally refer to SLM 18 as using reflectivetechnology, such as a DLP digital micromirror device as described above,it is contemplated that those skilled in the art having reference tothis specification will be readily able to adapt this invention for usewith transmissive SLM devices such as those incorporating thetechnologies described above.

Typically, SLM 18 will be controlled by graphics driver 14 in apulse-width-modulated manner, to precisely control the brightness oflight reflected from SLM 18 along the desired path to be displayed onscreen 12, for each primary color for each pixel. Incident light fromlight source 16 that is not to be part of the displayed image (i.e.,that light that is directed away for dark or darker pixels) ispreferably recycled for efficiency. In this manner, SLM 18 spatiallymodulates the light that is eventually projected onto screen 2, with themodulation being controlled according to the information in the image tobe displayed.

The light reflected from SLM 18 received by projection optics 20.Projection optics 20, as will be described in further detail below,preferably includes multiple telecentric lenses arranged in multiplegroups. The purpose of projection optics 20 is to provide a focusedpattern of light of the desired size and resolution upon aspheric mirror10. That focused pattern will, as mentioned above, reflect from asphericmirror 10 onto the backside of screen 12. Projection optics 20 alsocompensate and correct for aberrations in the light pattern, and thoseaberrations resulting from the shape of aspheric mirror 10. The detailedconstruction of projection optics 20 according to the preferredembodiments of the invention will be described in further detail below.

The functional arrangement of elements shown in FIG. 2 results in adisplayed image on screen 12 of high resolution, high contrast, andexcellent brightness. In addition, according to this preferredembodiment of the invention, the physical elements of the light source16, SLM 18, projection optics 20, and aspheric mirror 10 are arrangedand constructed to reduce the “chin” and depth dimensions of the displaysystem enclosure.

FIGS. 3 a and 3 b illustrate the construction of light source 16, andits arrangement with SLM 18 according to the preferred embodiment of theinvention. As will be apparent from the following description, lightsource 16 is a laser-based light source, as the attributes of laserillumination are especially beneficial in display system 15 according tothis preferred embodiment of the invention, for reasons discussed below.While it is contemplated that other light source types and arrangementsmay be used in connection with this invention, it is contemplated that alaser-based light source, such as light source 16 of FIGS. 3 a and 3 b,will be especially beneficial.

The arrangement of light source 16 as shown in FIG. 3 a is believed tobe conventional in the art. However, its construction and arrangement isdescribed herein, to provide context and understanding for the preferredembodiment of the invention described below.

Laser array 22 provides the light energy involved in the projection ofimages in display system 15, according to this embodiment of theinvention. In this example, laser array 22 includes one or more lines ofsolid-state lasers, for each of three or more colors. Typically, thethree “primary” colors of red, blue, and green are used in projectiondisplay systems; as such, laser array 22 includes one or more lines oflasers in an array for each of these colors. As suggested by FIG. 3 a,these arrays 22R, 22G, 22B (for red, green, blue, respectively) arespatially separated from one another, such that the collimatedmonochrome light from each array 22R, 22G, 22B travels in a plane,parallel but not coplanar with the light from the other arrays 22R, 22G,22B. The length of each of arrays 22R, 22G, 22B (i.e., the number ofsolid-state laser emitters in each) corresponds to the correspondingdimension of SLM 18, so that each array 22R, 22G, 22B can illuminate aportion of SLM 18 across its width (i.e., corresponding to the width ofthe projected image). The planes of collimated monochromatic light fromarrays 22R, 22G, 22B are directed by corresponding mirrors 23R, 23G,23B, respectively, to rotating mirror 24.

Rotating mirror 24, in this embodiment of the invention, is a rotatingmirror having multiple reflective surfaces. In this example, rotatingmirror 24 has a hexagonal cross-section, and is of sufficient length (inthe direction normal to the page of FIG. 3 a) to direct the entire widthof the output from each of arrays 22R, 22G, 22B. Mirror 25 re-directsthe reflected collimated light of each color from rotating mirror 24 toTIR prism 28. TIR prism 28 is a conventional “total internal reflection”prism element, which reflects or transmits incident light depending onwhether the angle of incidence exceeds or is less than a critical anglefor the material, as known in the art. As conventional in the art, TIRprism 28 is formed of two prism elements adjacent to each other, with asmall air gap between them. TIR prism 28 reflects the collimatedmultiple color light to different regions of SLM 18. In this manner, itis intended that the light of each of arrays 22R, 22G, 22B willilluminate one or mores in separate one-third regions of SLM 18. SLM 18is synchronously controlled, by graphics driver 14 as discussed above,to spatially modulate the collimated light of the appropriate primarycolor, according to the information in the image to be displayed. Themodulated light is then passed by TIR prism 28 to projection optics 20.

It is contemplated that those skilled in the optics art will be readilyable to design rotating mirror 24, and TIR prism 28, and the variousmirrors using conventional design techniques.

In operation, the rotation of rotating mirror 24 temporally “scrolls”colored light across SLM 18. For the position shown in FIG. 3 a, redlight from array 22R illuminates one or more rows of micromirrors in theupper third of SLM 18, green light from array 22G illuminates one ormore rows of micromirrors in the center third of SLM 18, and blue lightfrom array 22B illuminates one or more rows of micromirrors in the lowerthird of SLM 18. This situation is illustrated in FIG. 3 b by DMD state18(1). Upon rotating mirror 24 rotating in the direction illustrated,the particular rows of micromirrors illuminated by the collimated lightalso scroll across SLM 18. The order in which light is directed to SLM18 changes as a vertex of rotating mirror 24 passes below the point atwhich red light from array 22R impinges, after which the red light isreflected at a shallower angle, and is directed via mirror 25 and TIRprism 28 to one or more rows of micromirrors in the lower third of SLM18; meanwhile, the steeper reflection angle of the surface of rotatingmirror 24 at this time will direct green light to one or more rows ofmicromirrors in the upper third of SLM 18, and blue light to one or morerows of micromirrors in the center third of SLM 18, as shown in FIG. 3 bby DMD state 18(2). The scrolling continues in this manner, with bluelight eventually being directed to one or more rows of micromirrors inthe upper third of SLM 18, during which time red light is directed toone or more rows of micromirrors in the center third of SLM 18, andgreen light is directed to one or more rows of micromirrors in the lowerthird of SLM 18. The process then repeats.

As known in the art and as mentioned above, during such time as light ofa particular color is directed to micromirrors of SLM 18, graphicsdriver 14 synchronously controls the operation of those micromirrors toselectively modulate the incident light into or out of the light path tobe projected onto screen 2, depending upon the content of the image tobe displayed. In this manner, the operation of SLM 18 is controlled toinclude information in the light reflected through TIR prism 28 toprojection optics 20.

Projection optics 20, according to this preferred embodiment of theinvention, includes a rear group of glass lenses of spherical curvature,and a front group of aspheric plastic lenses, with one or more mirrorsdisposed within the lenses of these groups, as will be described below.As will be evident from the following description, the construction andarrangement of the lenses within projection optics 20 are a significantfactor in permitting the enclosure of display system 15 to have minimalchin and depth dimensions.

Referring now to FIG. 4 a, the construction and arrangement of reargroup 20 a of projection optics 20 will be described in detail,following the path of light in the direction from SLM 18 to screen 2. Afirst element of rear group 20 a is window 18W of SLM 18; as typical inthe art, window 18W is merely a transparent protective glass window, anddoes not affect the path of transmitted light. According to thispreferred embodiment of the invention, the light from both “on” pixelsand “off” pixels are transmitted by window 18W to the next element ofrear group 20 a, which is total internal reflection (TIR) prism 32.

As known in the art, TIR prism 32 serves to direct light into differentpaths, depending upon the angle of incidence of that light as comparedto a critical angle. TIR prisms, such as TIR prism 32, areconventionally used at the output of a DMD device, to pass desired lightbeams from those pixels that are to be displayed, and to reflect awaythe light beams from those pixels that are to be “dark”. TIR prism 32may be constituted as including TIR prism 28 of light source 16, with anadditional surface to reflect and transmit the “off” and “on” pixellight modulated by SLM 18, or alternatively may be a separate TIR prismfrom prism 28. In either case, the modulated light reflected from SLM 18is directed at a high incident angle such that it passes through the TIRsurface of the TIR prism and through the air gap therein. If the angleof incidence of a given light beam at the TIR surface of TIR prism 32 isless than the critical angle, it is reflected away as shown in FIG. 4 a(“OFF” PIXEL LIGHT). Modulated light at an angle of incidence greaterthan the critical angle is transmitted by TIR prism 32 along theprojection path, toward the lens elements 33 through 39 of rear group 20a.

Lenses 33 through 39 are each spherical glass lenses, arranged on theoptical axis of the light from SLM 18 through TIR prism 32. None oflenses 34 through 39 are tilted, and the aperture of none of theselenses is clipped. Details of the construction of lenses 33 through 39,according to this example of the preferred embodiment of the invention,are provided in Table 1:

TABLE 1 radius of element curvature thickness diameter (mm) (mm)material (mm) Lens surface ∞ 0.844 air 22.082 space from TIR 32 to lens33 28.436 9.2 S-NSL5 glass 23.3 lens 33 −16.705 2 S-LAH58 glass 23.3lens 34 −48.5 0.099 Air 24.6 space from lens 34 to lens 35 47.975 7.4S-FPL53 glass 24.9 lens 35 −29.832 0.07 air 24.9 space from lens 35 tolens 36 26.593 2 S-LAH64 glass 21.5 lens 36 15.455 2 air 19.2 space fromlens 36 to lens 37 23.713 7.5 S-NSL36 glass 20.2 lens 37 −15.073 1.9S-LAH58 glass 19.66 lens 38 −134.532 12.509 air 19.66 space from lens 38to lens 39 −317 5.018 S-NSL3 glass 19.4 lens 39 −24.178 64.778 air 19.4space from lens 39 to aspheric lens 42

As evident from FIG. 4 a, and as those skilled in the art will realizefrom the example of the lens construction of Table 1, lenses 33 through39 of rear group 20 a constitute a “telecentric” multi-element lensgroup, in that the chief rays for all points across the object definedby window 18W are parallel to the optical axis through lenses 34 through39. As known in the art, the optical properties of telecentricity arebeneficial in many ways. For purposes of this invention, one importantbenefit of telecentricity is that distortion due to position across theobject plane (window 18W) is eliminated. In addition, position of theobject plane (window 18W) relative to the lenses does not affect theimage size in a telecentric lens system, reducing the sensitivity of therelative position of these elements to one another and thus facilitatingmanufacture of the system. As such, the design of a telecentric lenssystem such as lenses 33 through 39 in this embodiment of the invention,in combination with a collimated light source such as laser light source16, reduces the sensitivity of projection optics 20 to defocus and tomisalignment of SLM 18.

However, as discussed above, the current trend in projection displaysystems is to use non-telecentric projection lens systems. Because thenon-parallel chief rays from the object plane converge in anon-telecentric lens system, the diameter of the projection lenses canbe greatly reduced, relative to the dimensions of the object plane. Thisenables “slower” (i.e., larger f-number) lenses to be used in theprojection lens system, which greatly reduces aberrations from thelenses themselves, reduces the complexity of the projection lens system,and greatly reduces the cost of the lenses themselves.

According to the preferred embodiment of this invention, however, theuse of laser-based light source 16 enables the construction of reargroup 20 a of projection optics 20 as a telecentric lens system, withinthe physical constraints of the enclosure of projection system 15 and atreasonable cost. The concept of “etendue” is useful in the optical art,and refers to the geometric or spatial capability of an optical systemto transmit or receive light. In the context of an SLM-based system, thesource of light incident on the SLM has an etendue that corresponds tothe size and directionality of the source; the SLM also has an etenduethat corresponds to its size and ability to receive light from variousdirections. As known in the art, lamp and LED light sources tend to havelarge source etendue values, relative to the SLM that is beingilluminated. This mismatch indicates that only a fraction of the lightemitted by the source is useful in the projection system. In otherwords, the light “cone” emitted by both lamp and LED light sourcessubtends a wide angle, relative to the aperture defined by the SLM. Assuch, for a desired brightness level, the lamp or LED light source isrequired to be of relatively high power.

In contrast, a laser-based light source, such as light source 16discussed above, has a relatively low source etendue level, for exampleon the order of 100 times smaller than that of a conventional lamp orLED light source. As such, the source etendue of laser light source 16is preferably well-matched to the etendue of SLM 18, in projectionsystem 15 according to this preferred embodiment of the invention.

From an optical standpoint, the lower source etendue of laser-basedlight source 16 corresponds to a narrower angle of the incident light“cone”. In other words, the effective diffraction “aperture” defined byeach pixel of SLM 18 is narrower for light from a lower etendue source,such as laser-based light source 16, than it is for a higher etenduesource, such as a lamp or LEDs. It has been observed, in connection withthis invention, that this smaller or narrower aperture permits “slower”lenses (i.e., lenses with higher f-numbers) for optical correction andfocus to be used, for a given resolution. This effect of low sourceetendue on the f-number of the projection lenses compensates for thelower f-number lenses, and more complex arrangement of such lenses, thatare required to realize a telecentric lens system. For example, thef-number of rear group 20 a of projection optics 20, constructed toinclude lenses 33 through 39 as described above, is about f/2.8 orhigher (i.e., slower), for the case of a laser-based light source 16 anda DMD-based SLM 18 measuring 0.45″ in width. It has been demonstratedand observed, in connection with this invention, that the use oflaser-based light source 16 enables such relatively slow f-number opticsas rear group 20 a, in consideration with other factors such asgeometrical lens aberration (i.e., spherical aberration that isdependent on aperture) on one hand, and MTF diffraction effects that“blur” the pixel resolution from slower lenses, on the other hand. Assuch, the use of laser-based light source 16, as described above,enables rear group 20 a of projection optics 20 to be telecentric atvirtually no cost, while keeping the lens speed and size reasonable, andwhile permitting the “throw” distances of the projection lenses to alsobe modest. Indeed, it is believed that the use of a lamp or LED lightsource with the telecentric projection lens rear group 20 a would notresult in a projected image of optimum quality (i.e., adequate qualityfor today's high-definition television marketplace), without greatlyincreasing the throw distances of the lenses beyond the desired size ofthe enclosure of projection system 15. Accordingly, the combination oflaser-based light source 16 with the telecentricity of rear group 20 aof projection optics 20, provides important advantages in theconstruction of a mirror-based projection display system, especially inthe form factor of such a system as will be described below.

Alternatively, light source 16 may be realized by way of one or morelight-emitting diodes (LEDs), for example one or more LEDs for each ofthe primary colors. Conventional LED-based light sources or “engines”are known that provide one LED for each of the red, green, and blueprimary colors, or an array of LEDs for each primary color (e.g., sixLEDs for each of red, green, and blue). As mentioned above, the sourceetendue of an LED-based light source 16 is greater than a laser-basedsource, which requires a wider aperture for rear group 20 a ofprojection optics 20, and perhaps a larger SLM 18. For example, it iscontemplated that use of an LED-based light source 16 will require thef-number of rear group 20 a to be f/2.0, for a DMD-based SLM 18measuring 0.65″ in width. But even with this additional constraint onrear group 20 a, it is contemplated that an LED-based light source 16may be implemented in projection system 15, according to this invention,while still providing an enclosure with low “chin” and “depth”measurements as will be described below.

Referring back to FIG. 4 a, the light for “on” pixels that istransmitted through lenses 33 through 39 is then directed at opticalactuator 40. Optical actuator 40 is a fully-reflective plane mirror thatredirects the path of the light projected from last lens 39. Accordingto the preferred embodiment of the invention, optical actuator 40 isslightly “dithered” between two angles relative to the optical axis oflenses 33 through 39. In this regard, it is contemplated that opticalactuator 40 includes a motor or other mechanism for controllablypositioning its reflective surface at a selected one of at least twodifferent angles, relative to the optical axis of rear group 20 a. It iscontemplated that this motor or mechanism will be controlled bycircuitry within projection system 15, for example by graphics driver 14itself, or by other circuitry that is synchronized to graphics driver14.

As known in the art in connection with the SMOOTH PICTURE™ technologydeveloped and available from Texas Instruments Incorporated,odd-numbered image pixels can be assigned to one subframe of an imageframe, and even-numbered image pixels can be assigned to a secondsubframe. The timing control signals applied to SLM 18 can be similarlydivided. In displaying the image, optical actuator 40 is placed at oneangle relative to the optical axis of lenses 34 through 39 for onesubframe, and is placed at a second angle relative to the optical axisfor the next subframe; the angles of optical actuator 40 are selected sothat the difference between these two positions, in projected light pathat screen 2, is about one-half pixel width. Typically, the pixels of SLM18 are diamond-shaped, such that the light beam or ray from a givenpixel is shifted in the direction orthogonal to that defined by opticalactuator 40, also by one-half the pixel width. As such, optical actuator40 not only directs the projected light along its path in a differentdirection from that of lenses 34 through 39, but also implements theSMOOTH PICTURE™ technology so that the resulting resolution of thedisplayed image is greatly improved.

Projection optics 20 of projection system 15, according to thisembodiment of the invention, also includes front group 20 b of lenses.FIG. 4 b illustrates the optical arrangement of front group 20 b; aswill be described below, the physical arrangement of front group 20 bdiffers from its effective optical path, for purposes of minimization ofdepth.

As shown in FIG. 4 b, front group 20 b includes three aspheric elements,namely aspheric meniscus lenses 42, 44, and aspheric mirror 10.According to the preferred embodiment of this invention, as shown inFIG. 4 b, each of these aspheric elements is constructed of opticalacrylic plastic. This permits each of aspheric elements 42, 44, 10 to bephysically “clipped” at or near its optical axis of these elements,because the optical path utilizes only a portion of the entire asphericsurface. This greatly facilitates the positioning of these asphericelements within the enclosure of display system 15. In addition, as willalso be described in further detail below, sufficient space is providedbetween aspheric lens 42 and aspheric lens 44 for a two-surface foldingmirror, which will bend the light path back on itself to save additionalform factor volume.

According to this embodiment of the invention, aspheric lenses 42, 44are constructed to operate as a medium-to-wide angle projection lenssystem, which reduces the magnification required of aspheric mirror 10.The detailed construction of aspheric elements 42, 44, 10 will now bediscussed in connection with the diagram of FIG. 4 c, in which each ofthese elements 42, 44, 10 are shown in an “unclipped” form, for clarityin the description of their construction.

According to the preferred embodiment of the invention, each of elements42, 44, 10 are formed of optical acrylic plastic, having surfaces thatare each defined as a rotationally or axially symmetric polynomialaspheric surface. These surfaces can be described by way of a polynomialexpansion of a deviation from a surface that is spherical, or asphericdefined as a conic (i.e., nearly “axiconic”). In this example, thesurfaces of elements 42, 44, 10 are “even” asphere surfaces, as onlyeven powers of the polynomial expansion are used (odd powers are zeroed)because of the axial symmetry. The lateral distance, or surface “sag”, zfrom the front apex of the surface to a radial point r from the vertexof the aspheric surface along the optical axis, is commonly definedaccording to

$z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {a_{1}r^{2}} + {a_{2}r^{4}} + {a_{3}r^{6}} + {a_{4}r^{8}} + {a_{5}r^{10}} + {a_{6}r^{12}}}$where k is the “conic coefficient” for the surface, and where c is thecurvature (1/radius) of the base sphere (from which the aspheredeviates) at the vertex, and the coefficients a_(i) are the asphericcoefficients defining the shape of the asphere.

Specific values used to define the surfaces of elements 42, 44, 10 in anexample of the preferred embodiment of the invention are specified inTable 2, in which the lens surfaces 42S1, 42S2, 44S3, 44S4 refer to thesurfaces of lenses 42, 44 as shown in FIG. 4 b, and in which the surface10S5 refers to the substantially axiconic reflective surface of asphericmirror 10:

TABLE 2 clear aperture output C (reciprocal of lens (~physical radius ofcurvature radius of surface radius, in mm) (mm) curvature) (mm⁻¹) k 42S121.4 20.814 0.04804454 −7.435999 42S2 26.3 24.102 0.04148992 −0.309590144S3 51.6 17.453 0.05729674 −9.0506895 44S4 54.7 30.081 0.03324395−12.303 10S5 91.3 −13.516 −0.07398622 −2.887278 lens surface a₁ a₂ a₃ a₄a₅ a₆ 42S1 0 −3.712239E−05  2.621195E−07 −5.760937E−10 8.906940E−13−3.383627E−16 42S2 0 −9.963297E−05  3.197044E−07 −5.579417E−105.586180E−13 −2.430105E−16 44S3 0 1.732750E−05 −2.334335E−08  1.638530E−11 −5.311442E−15   6.322489E−19 44S4 0 1.604374E−05−1.786992E−08   1.062825E−11 −3.009051E−15   3.174648E−19 10S5 09.390947E−08 7.529221E−12 −3.619127E−15 4.031248E−19 −1.584210E−23It is contemplated that those skilled in the art having reference tothis description will be able to readily construct aspheric lenses 42,44, and aspheric mirror 10 as suitable for a particular systemapplication, without undue experimentation. In this example, the opticalpath length from lens 39 to aspheric lens 42, via optical actuator 40,is about 64.78 cm, the optical path length from aspheric lens 42 toaspheric lens 44, via folding mirror 48, is about 53 cm, and opticalpath length from aspheric lens 44 to aspheric mirror 10 is about 22 cm.Each of these paths is through air. FIG. 4 b illustrates the generalpaths of projected light through lenses 42, 44, as reflected by asphericmirror 10 to screen 2.

According to this preferred embodiment of the invention, it iscontemplated that the magnification power of aspheric lenses 42 and 44,in combination is greater than 1.0. For the example of aspheric lenses42, 44 described above, the magnification power of these two lenses isshown by curve 50 of FIG. 8 a, as varying between about 2.25 and 1.25over normalized field of view (FOV) values ranging between about 0.1 and1.0, with the minimum magnification at about 0.5 of FOV, normalized.FIG. 8 c illustrates the definition of normalized field of view, rangingfrom the radial distance at the optical axis of the lens or mirrorelement LME (having a normalized FOV value of 0.0) to the point on thesurface of the lens or mirror corresponding to the pixel at the diagonalcorner of the image displayed on screen 2 (normalized FOV of 1.0).This >1.0 magnification power of aspheric lenses 42, 44 reduces themagnification power required of aspheric mirror 10 according to thispreferred embodiment of the invention, relative to conventional systems.Curve 52 of FIG. 8 b illustrates the magnification of aspheric mirror 10over normalized FOV between about 0.1 and 1.0, as varying from about15.0 to about 3.5; it is contemplated that this magnification byaspheric mirror 10 is substantially less than that of conventionalprojection systems using a substantially larger aspheric mirror thanaspheric mirror 10 in this embodiment of the invention, suchconventional systems generally including a single aspheric lens elementthat does not substantially magnify the image. The arrangement ofplastic aspheric lenses 42, 44 according to this embodiment of theinvention results in this smaller size for aspheric mirror 10, becausethe complexity of the system of aspheric lenses 42, 44 defines anoptimum distance between aspheric mirror 10 and rear group 20 a; thisoptimum distance does not necessarily impact the depth of the enclosure,but enables the smaller size for front aspheric mirror 10.

In addition, it has been observed, in connection with this invention,that conventional projection systems involving an aspheric projectionmirror will typically have only the final aspheric lens element formedof an optical plastic, with all other lens elements formed in glass. Incontrast, the arrangement of this invention enables both aspheric lenses42, 44 to be formed of optical acrylic plastic, reducing the cost of thesystem and also permitting “clipping” of these lenses in the mannerillustrated in the Figures and described herein.

By way of further description, curve 54 of FIG. 8 b illustrates thetotal magnification of the “front group” of aspheric lenses 42, 44 incombination with aspheric mirror 10. While the magnification at lownormalized FOV points are greatly magnified by these aspheric elements42, 44, 10, it has been observed, in connection with this invention,that projection optics 20, including aspheric mirror 10, has outstandingdistortion performance. FIG. 8 d illustrates the distortion exhibited byan example of projection system 15 according to this preferredembodiment of the invention, as measured over normalized FOV from 0.10to 1.00 (bottom to top along the y-axis). As evident from this plot, theoverall distortion of the system remains within 1.00% over the entireimage range, which is excellent performance for this scale of opticalsystem.

As discussed above relative to FIG. 4 b, aspheric lenses 42, 44, andaspheric mirror 10 are physically clipped to save volume within the formfactor of the enclosure of projection system 15. This clipping resultsin the surface cross-section of aspheric lenses 42, 44, and asphericmirror 10 residing substantially within a single half-plane, as shown inFIG. 4 b. In addition, also as discussed above relative to FIG. 4 b, thedistance between aspheric lenses 42, 44 is selected to be sufficient toinsert a two-panel folding mirror between these elements, to direct thelight path in an efficient manner within the enclosure of projectionsystem 15. Referring now to FIG. 5 a, the arrangement of rear group 20 aand front group 20 b of projection optics 20, according to the preferredembodiment of the invention, will now be described.

FIG. 5 a is a perspective view of projection optics 20, includingfolding mirror 48, which has two surfaces 48 a, 48 b, and includingscreen 2 (a portion of which is shown in phantom). As illustrated inFIG. 5 a. rear group 20 a is oriented so that its optical axis isgenerally parallel to the plane of screen 2, with the path of itsprojected light diverted substantially perpendicularly by opticalactuator 40. The precise angular relationship of this optical axis toscreen 2 is not important, as the light path is controlled by opticalactuator 40. However, the arrangement of rear group 20 a to be generallyparallel to the plane of screen 2 permits the depth of an enclosure fordisplay system 15 to be minimized. Aspheric lens 42 is positioned sothat its optical axis is generally in the perpendicular plane relativeto screen 2, and receives the projected light as reflected by opticalactuator 40.

Folding mirror 48 is constructed as two planar reflective panels thatare at a selected angle (generally perpendicular) relative to oneanother, and that are disposed in the light path between aspheric lens42 and aspheric lens 44. Aspheric lens 42 is oriented in substantiallythe reverse direction, relative to aspheric lens 42 from the opticalarrangement of FIG. 4 b. As shown in FIG. 5 a, the presence of foldingmirror 48 enables aspheric lens 44 to reside substantially aboveaspheric lens 42, within the physical arrangement of projection system15. Aspheric lens 44 is aimed at aspheric mirror 10, which in turn ispositioned to direct projected light to screen 2.

FIG. 6 illustrates the light path from optical actuator 40 to screen 2.In this arrangement, the light projected from rear group 20 a andreflected by optical actuator 40 is then transmitted by aspheric lens 42toward bottom panel 48 b, and reflected from bottom panel 48 b to toppanel 48 a, from which the light is reflected to aspheric lens 44.Aspheric lens 42 magnifies the image of the projected light, as evidentfrom the diverging light rays illustrated in FIG. 6. The light reflectedfrom top folding mirror panel 48 a is then further magnified by asphericlens 44, and projected onto the surface of aspheric mirror 10, which inturn reflects the projected light toward screen 2. As discussed aboverelative to FIGS. 8 a and 8 b, it is contemplated that the magnificationapplied by aspheric lens 44 (and, to a lesser extent, by aspheric lens42) reduces the curvature and magnification of aspheric mirror 10,improving the overall resolution and fidelity of the projected image.

FIG. 5 b illustrates the physical arrangement of projection optics 20within projection system 15 in a top-down view, further illustrating thephysical relationship of aspheric lenses 42, 44 to one another, and tothe other elements. FIG. 5 c is a perspective view of these elementsfrom the opposite direction from that shown in FIG. 5 a, and furtherillustrates the reflecting surface of optical actuator 40. And FIG. 5 dis an elevation view from the same side as shown in FIG. 5 a, but in adirection that is substantially parallel to the plane of screen 2.

As evident from FIGS. 5 a through 5 d and FIG. 6 and the abovedescription, the arrangement of projection optics 20 and aspheric mirror10 enables the enclosure of these elements within a volume that iscompetitive with LCD and plasma display systems, especially inconnection with the important dimensions of the “chin” and depth of thesystem enclosure. FIG. 7 illustrates the relative position of projectionoptics 20 and aspheric mirror 10, in connection with a rear view ofdisplay system 15. In actual implementation, enclosure 50 (shown inshadow in FIG. 7) surround the system components, including screen 2 andprojection optics 20, aspheric mirror 10, light source 16 (not shown)and the other elements of projection system 15.

In this example, screen 2 is a 44-inch widescreen (16:9 aspect ratio)projection screen, upon which projection optics and aspheric mirror 10are capable of projecting a full resolution image. It is contemplatedthat enclosure 50 can provide sufficient volume for the elements ofprojection system 15 in a manner that is quite efficient. For thisexample, and given the example of the construction described above, itis contemplated that enclosure 50 for this 44-inch system 15 can containthese elements within a “chin” dimension (from the bottom of screen 2 tothe base of enclosure 50, as shown) of 6 inches or less (and an“optical” chin dimension, corresponding to the vertical offset from theoptical axis of aspheric mirror 10 to the bottom of screen 2, of 4inches or less), and a depth (from the front of screen 2 to the rear ofthe enclosure) of about 6 inches or less. These dimensions, for a44-inch projection display system 15, are similar to current-day LCD andplasma display systems available in the marketplace.

The minimal chin and depth dimensions are attained by projection system15, while meeting other important constraints in the design of thesystem. One important constraint that is met by projection system 15according to this preferred embodiment of the invention is the minimumangle (from the normal) of incident light reflected from aspheric mirror10 to screen 2. As known in the art and as described above, this minimumangle of incidence is required of TIR Fresnel screens such as screen 2,in order to eliminate flare and non-uniform contrast in the projectedimage. According to this preferred embodiment of the invention, evenwith the minimal chin and depth measurements specified above, theminimum angle of incidence φ_(m), at the worst case location of thebottom of screen 2, is greater than 50°.

Projection system 15 constructed according to this preferred embodimentof the invention is capable of being housed in enclosure 50 of thissmall form factor, while projecting an image of excellent resolution. Ithas been observed, according to this invention, that projection optics20 in projection system 15 provide excellent (>50%) response even athigh spatial frequencies (>0.65 cycles per mm), over the screen 2. Inaddition, because of the telecentricity of rear group 20 a of projectionoptics 20, excellent response is maintained over relatively wide focusshifts. Distortion and lateral color shift are also minimized accordingto this design. As such, projection system 15 according to thisembodiment of the invention is fully capable of accurately and preciselyprojecting modern “high definition” images.

In addition, it has been discovered and observed that this inventionprovides the additional important benefit of greatly improved stabilityover variations in temperature and humidity in the system environment.It has been observed that conventional projection systems, for exampleDMD-based projection systems using only an aspheric mirror, suffer fromloss of resolution over variations in temperature and humidity, due tothe effects of such environmental variations on the aspheric mirror.FIGS. 9 a through 9 c illustrate the modulation transfer function (MTF),expressed as the modulus of the DTF ranging from 0.0 to 1.0, overspatial frequency in cycles per millimeter, for a conventional singleaspheric mirror system at temperatures of −5° C., +20° C., and +45° C.,respectively. As shown in FIG. 9 b, the resolution performance is quitegood for this conventional system; however, the higher and lowertemperature performance is dramatically poorer, with poor resolution (aspoor as 0.13 cycles per mm) exhibited for these temperatures

On the other hand, projection system 15 according to this preferredembodiment of the invention provides relatively good stability overtemperature, as exhibited by FIGS. 9 d through 9 f at temperatures of−5° C., +20° C., and +45° C., respectively. As evident from these plots,the degradation in resolution over temperature is much reduced forprojection system 15, relative to the conventional system for whichperformance is shown in FIGS. 9 a through 9 c. It is believed, accordingto this invention, that this improved stability is because any thermaldilation of plastic aspheric mirror 10 will be compensated bysubstantially equivalent thermal expansion of plastic aspheric plasticlenses 42, 44, which presents the opposite effect of the dilation ofaspheric mirror 10. Conventional systems using only a single plasticaspheric mirror do not have this compensating effect, resulting in thepoor thermal performance illustrated in FIGS. 9 a through 9 c.

It is also contemplated that, because of this improved thermalstability, the plastic aspheric lenses 42, 44 and plastic asphericmirror 10 of projection system 15 according to this invention can beconstructed to be stable over temperature in a system using a plasticbaseplate, thus reducing manufacturing cost and also reducing mechanicalstress due to thermal mismatch between the plastic lenses and thebaseplate.

Furthermore, the nature of DMD-based projection systems such as displaysystem 15 according to this preferred embodiment of the invention lendsitself well to scaling to larger screen sizes. As such, it iscontemplated that the ratio of chin and depth of enclosure 50, to thesize of screen 2, will be the same or better as the size of screen 2 isscaled upward. These larger screen display DMD-based display systems arecontemplated to be less expensive than corresponding LCD and plasmasystems, given the scalability of the DMD projection engine relative tothose other technologies.

While the present invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

1. A projection display system, comprising: a light source for producinglight of at least one primary color; a graphics driver, for generatingcontrol signals according to a sequence of images to be displayed; aspatial light modulator for modulating the light of at least one primarycolor responsive to control signals from the graphics driver; projectionoptics, comprising: a rear group of lenses of spherical curvature,arranged as a telecentric lens group for modulated light received fromthe spatial light modulator; a first aspheric lens, receiving light fromthe rear group of lenses; a folding mirror comprising two planarsurfaces, for redirecting light from the first aspheric lens; and asecond aspheric lens, receiving light from the folding mirror and havinga magnification greater than 1.0; an aspheric mirror, for reflectinglight received from the second aspheric lens; and a projection screen,positioned to receive the reflected light from the aspheric mirror. 2.The projection display system of claim 1, wherein the second asphericlens is disposed generally above the first aspheric lens; and whereinthe folding mirror is positioned so that the light transmitted throughthe second aspheric lens is traveling in a direction substantiallyopposite to that of light transmitted through the first aspheric lens.3. The projection display system of claim 1, wherein the first andsecond aspheric lenses are formed of an acrylic plastic.
 4. Theprojection display system of claim 3, wherein the second aspheric lenshas a surface cross-section that corresponds to an even asphere.
 5. Theprojection display system of claim 4, wherein the aspheric mirror has asurface cross-section that corresponds to an even asphere.
 6. Theprojection display system of claim 5, wherein the second aspheric lensand the aspheric mirror are clipped so that their surface cross-sectionsare confined to substantially within a single half-plane.
 7. Theprojection display system of claim 1, further comprising: a mirror fordirecting light from the rear group of lenses to the first asphericlens.
 8. The projection display system of claim 7, further comprising:an optical actuator mechanism for pivoting the mirror to direct light ata selected angle; and circuitry for controlling the optical actuatormechanism synchronously with the control signals applied to the spatiallight modulator.
 9. The projection display system of claim 7, whereinthe rear group of lenses defines an optical axis that is generallyparallel to the plane of the projection screen; and wherein the mirrordirects light from the rear group of lenses in a generally perpendiculardirection.
 10. The projection display system of claim 1, wherein thelight source comprises at least one laser.
 11. The projection displaysystem of claim 10, wherein the light source comprises: a plurality ofsolid-state laser arrays, each solid-state laser array generatingcollimated light of one of a plurality of primary colors incident on aportion of the spatial light modulator; and wherein the spatial lightmodulator modulates the light of each of the plurality of primary colorsresponsive to control signals from the graphics driver.
 12. Theprojection display system of claim 1, wherein the light source comprisesat least one light-emitting diode.
 13. The projection display system ofclaim 1, wherein the rear group of aspheric lenses defines a lens systemhaving an f-number of about f/2.8 or slower.
 14. The projection displaysystem of claim 1, further comprising: an enclosure, housing the lightsource, spatial light modulator, projection optics, and projectionscreen.
 15. The projection display system of claim 14, wherein theaspheric mirror is disposed within the housing at a locationsubstantially below and behind the projection screen.
 16. The projectiondisplay system of claim 15, wherein the projection screen comprises aTIR Fresnel screen; and wherein light is reflected from the asphericmirror to the projection screen at an angle of incidence of at least 50°from normal to the projection screen.
 17. The projection display systemof claim 15, wherein the aspheric mirror has a magnification power ofless than about 15.0 over a normalized field of view from between about0.1 and 1.0.
 18. The projection display system of claim 1, wherein thespatial light modulator comprises an array of digital micro-mirrors. 19.The projection display system of claim 1, wherein the spatial lightmodulator is of a type selected from a group consisting of liquidcrystal on silicon (LCOS) spatial light modulators, high temperaturepolysilicon (HTPS) spatial light modulators, transmissive LCD spatiallight modulators, and diffractive 1-D spatial light modulators.